1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium, and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may include a fixing belt or a fixing film to apply heat to the recording medium bearing the toner image. FIG. 1 is a sectional view of a fixing device 140R1 including a fixing belt 5. The fixing belt 5 is looped around a heating roller 2 and a fixing roller 3 in a state in which a tension roller 4 biases the fixing belt 5. A pressing roller 6R presses against the fixing roller 3 via the fixing belt 5 to form a fixing nip between the pressing roller 6R and the fixing belt 5. The fixing belt 5 is heated by a heater 1 provided inside the heating roller 2. As a recording medium 7 bearing a toner image passes between the fixing roller 3 and the pressing roller 6R on the fixing belt 5, the fixing belt 5 and the pressing roller 6R apply heat and pressure to the recording medium 7 bearing the toner image to fix the toner image on the recording medium 7.
One problem with such an arrangement, however, is that the heating roller 2 has a relatively large heat capacity, resulting in a longer warm-up time for the fixing device 140R1. To address this problem, instead of the fixing belt 5 the fixing device may include a fixing film having a relatively small heat capacity. FIG. 2 is a sectional view of a fixing device 140R2 including a fixing film 14. A ceramic heater 13 is provided inside a loop formed by the fixing film 14 and supported by a stay 12 via a holder 11. The pressing roller 6R presses against the ceramic heater 13 via the fixing film 14 to form a fixing nip between the pressing roller 6R and the fixing film 14. As a recording medium bearing a toner image passes between the pressing roller 6R and the fixing film 14, the fixing film 14 heated by the ceramic heater 13 and the pressing roller 6R apply heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium.
However, the fixing film 14 also has a drawback in that, over time, friction between the ceramic heater 13 and the fixing film 14 sliding over the ceramic heater 13 increases, resulting eventually in unstable movement of the fixing film 14 and increasing the required driving torque of the fixing device 140R2.
To address the above-described problems, instead of the ceramic heater 13 the fixing device may include a hollow cylindrical heating member stationarily provided inside the loop formed by a fixing member (e.g., the fixing belt or the fixing film) across a clearance therebetween. A heater provided inside the hollow cylinder of the heating member heats the heating member, which in turn heats the fixing member, to maintain the fixing member at the proper temperature. The pressing roller is pressed against a nip formation member provided inside the loop formed by the fixing member via the fixing member to form the fixing nip between the fixing member and the pressing roller.
However, the heating member described above also has a drawback in that pressure applied by the pressing roller bends the nip formation member and the heating member supporting the nip formation member in such a manner that the nip formation member is sandwiched between the heating member and the fixing member. Specifically, the pressure applied by the pressing roller bends the center portion of the nip formation member in the longitudinal direction of the nip formation member parallel to the axial direction of the fixing member substantially. Accordingly, the heating member supporting the nip formation member is also bent. To address this problem, the center portion of the nip formation member in the longitudinal direction of the nip formation member may be given a convex shape protruding inward toward the heating member. However, when the pressing roller applies substantial pressure to the nip formation member having the convex center portion, the heating member contacted by the nip formation member is deformed like a bow along the convex shape of the nip formation member. Specifically, the center portion of the heating member in the axial direction of the fixing member, which is pressed by the convex center portion of the nip formation member, moves closer to the inner circumferential surface of the fixing member at the position diametrically opposite the fixing nip. By contrast, the lateral end portions of the heating member in the axial direction of the fixing member move away from the inner circumferential surface of the fixing member. In other words, the clearance provided between the fixing member and the heating member becomes excessively large at the lateral ends of the fixing member in the axial direction of the fixing member. Accordingly, heat is not transmitted from the heating member to the fixing member effectively at the lateral ends of the fixing member. As a result, the fixing member is not heated quickly, increasing the warm-up time of the fixing device.